Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems for eliminating noise from seismic data acquired at different times at the same location (4D seismic surveys), thereby achieving a clearer image of real changes occurring in the subsurface over time.
Discussion of the Background
A widely used technique for searching for oil or gas is the seismic exploration of subsurface (i.e., geophysical structure). Seismic exploration consists of generating seismic waves directed toward the subsurface, gathering data on reflections of the generated seismic waves from interfaces between layers of the subsurface, and analyzing the data to generate a profile (image) of the geophysical structure, i.e., layers of the investigated subsurface. Seismic exploration is used for exploring both land and subsurface under the sea floor.
However, collected seismic data includes noise, which may be of different types, e.g., coherent and incoherent. One example of such noise is produced by multiples. Multiples are known in the art to include waves that reflect more than once before being recorded by seismic receivers. For example, FIG. 1 shows a marine acquisition system 100 that includes a vessel 102 towing a seismic source 104 and a streamer 106. Streamer 106 has one or more seismic receivers 108. A wave 120a emitted by seismic source 104 may reflect from an interface 122 of interest and then be recorded by seismic receiver 108. However, it is also possible to emit a wave 120b that reflects a first time from the interface of interest 122 and a second time from the water surface 124 prior to be being recorded by the seismic receiver. In another example, a wave 120c is reflected from an interface not of interest 124, and then it reflects at least two more times, from the interface of interest 122 and from another interface, e.g., interface 124, before being recorded by the seismic receiver. These different waves have acquired dedicated names, for example, wave 120a is called primary, wave 120b is called ghost, and wave 120c is called multiples. While primary 120a is desired for processing, the ghost and multiples are also recorded by the seismic receivers, and these waves are considered noise, i.e., they hide the interfaces of interest.
There are traditional methods for removing multiples. Such methods may include, as illustrated in FIG. 2, a step 200 of receiving various vintages for a given subsurface, a step 202 of determining the multiples for each vintage, a step 204 of subtracting from each vintage a corresponding multiple, and a step 206 of generating 4D data based on the corrected vintages.
However, this method affects each vintage (i.e., 3D seismic data) individually and may introduce spurious events into the 4D image of the subsurface, which is undesirable. Thus, there is a need in the industry to develop new methods that reduce the number of spurious events and also remove noise (e.g., multiples) from recorded seismic data.